Dynamic braking systems have been known for some time and offer the advantages of saving wear of friction brake components and also, if the dynamic brake is of the regenerative type, of recovering some of the kinetic energy of the vehicle. A locomotive typically has an engine-generator unit adapted for supplying electrical power to a plurality of traction motors which drive associated drive axles and drive wheels. However, during dynamic braking, the motors are reconfigured to function as alternators for dissipating power through a resistor grid. The power dissipation produces a retarding force which works against the locomotive drive axles thereby slowing the locomotive.
More specifically, during dynamic braking the fields of the drive motors are serially connected to the generator for receiving a current therefrom, hereinafter actual field current. Furthermore, the armatures of the drive motors are electrically connected in series with a resistor grid formed of a plurality of high power resistors and mechanically connected to respective drive axles (not shown) for rotation therewith. The level of current flowing through the resistor grid, hereinafter referred to as actual grid current, is a function of the rotational speed of the armatures and the level of the actual motor field current. The power dissipation in the resistor grid causes a retardation force to act against the turning locomotive wheels, thereby supplementing the locomotive mechanical brakes (not shown). As should be apparent, the amount of power dissipation, and thus the amount of dynamic braking, is a function of the actual grid current.
Most control strategies regulate the level of dynamic braking by operating the engine at a constant speed and controlling the level of exciter current applied to the generator. This in turn controls the current output by the generator and, more specifically, the actual field current. The actual field current controls the magnitude of the actual grid current and thus the level of dynamic braking. However, since the amount of gain between the actual field current and the grid currents is a function of locomotive speed, accurate brake regulation is difficult to achieve.
More specifically, actual field current is related to the generator exciter current by the equation IFA=k.sub.1 * IE, where IFA represents the actual field current k.sub.1 is a constant and IE represents the exciter current applied to the generator. However, the actual grid current is a function of both locomotive speed and the actual field current. The level of grid current can be expressed with the following equation: IGA=k.sub.2 * RPM * IFA, where IGA represents the actual grid current, k.sub.2 is a constant, and RPM represents locomotive speed. By substitution, the actual grid current can be expressed as IGA=k.sub.3 * IE, where k.sub.3 is a constant accounting for conversion factors. As can be seen from the above equations, the actual grid current can be regulated by controlling the generator exciter current. However, the gain between the exciter current and the actual grid current varies with locomotive speed.
Past systems have employed speed sensors to determine locomotive speed and then used the detected speed to calculate the gain between actual field current and actual grid current. The calculated gain in turn is used to determine the generator exciter current required to achieve the desired dynamic braking level. These speed sensors add extra cost to the dynamic brake system and, therefore, the elimination of such sensors is desirable.
The present invention is directed towards addressing the above mentioned problems by providing a dynamic brake controller which automatically adapts to changing locomotive speed without requiring speed sensors. Other aspects, objects and advantages can be obtained from a study of the drawings, the disclosure, and the appended claims.